A card is selected and lost in the deck. Now the magician announces that he has an envelope with a prediction he proceeds to show to the audience. The deck is hold now by the spectator. The magician tosses the envelope into the air and it turns magically into the selected card.
Now the deck is ribbon spread and in the place of the card they find the envelope that contains a small duplicate of the same card selected by the spectator.
It is very easy to do.